Continuous-wave (CW) monochromatic lights at a number of wavelengths such as blue (λ˜490 nm) and orange (λ˜590 nm), which are useful for scientific research and medical or industry applications, cannot be directly generated from a laser diode or diode-pumped solid-state (DPSS) laser. As a consequence, traditional air-cooled Argon ion lasers, HeNe lasers, and dye lasers, though bulky and inefficient, have been the workhorse for the past years and are still playing important roles in these spectral ranges.
It is therefore highly desirable to develop compact and efficient solid-state lasers as their replacement.
In the past decade, a number of efforts have been devoted to the development of blue, green, or orange lasers by the use of indirect methods. The most commonly used method is based on frequency doubling technology. When a light of appropriate wavelength, e.g. 980 nm emanating from a laser diode, passes through a nonlinear crystal or waveguide, its second harmonic is generated due to the light-material interaction, resulting in doubled frequency, i.e., 490 nm. The frequency doubling can be realized through a direct single pass or through external cavity resonance with periodically poled nonlinear crystals or nonlinear waveguides. Lasers based on such approaches can be made extremely compact and highly efficient. In fact, some commercially available products have been developed to replace the legacy ion lasers. However, these lasers are generally expensive and may have long-term reliability issues, especially when costly waveguide materials are employed.
An alternative technology for wavelength conversion is based on Sum Frequency Mixing (SFM). As an example, in U.S. Pat. No. 5,345,457, Zenzie and Moulton demonstrated a dual-wavelength laser system with intracavity sum-frequency mixing. A Brewster prism assembly or a dichroic mirror was employed for combining the two input beams. As another example, in U.S. Pat. No. 5,802,086, Hargis and Pessot have investigated monolithic DPSS microlasers based on intracavity optical frequency mixing. However, these systems generally have shortcomings of complicated structure, high loss, and unstable operation. In addition, the achievable wavelengths in dual-wavelength lasers are limited to transitions with similar stimulated emission cross sections. For example, lasers at the orange wavelengths near 590 nm can be generated in Neodymium ion doped crystals by intracavity SFM because the ratio of the stimulated emission cross-sections for transition 4F3/2→4I11/2 and transition 4F3/2→4I13/2 is nearly one. In fact, both CW and Q-switched lasers at 593 nm through intracavity sum frequency mixing of 1064 nm and 1342 nm have been demonstrated by Chen et al. in publications appeared in Optics Letters Vol. 27, No. 6 and No. 20. More recently, Momiuchi et al. in U.S. Pat. No. 6,816,519 described generation of 593 nm laser through SFM of 1064 nm and 1342 nm. In their devices, the two fundamental lights were generated in separated resonant cavities with separated pumping sources and combined in a common nonlinear optical medium for wavelength conversion. The drawbacks of these devices include intracavity loss due to insertion of wavelength separating plates and stringent coating requirements.
In principle, laser output at blue spectral region near 488 nm to 492 nm can also be obtained from SFM between, e.g., well established 1064 nm and 914 nm lines. These lines can be generated from neodymium doped lasers such as Nd:YVO4 and Nd:GdVO4. This scheme, however, cannot be simply realized in a dual-wavelength laser because the transition 4F3/2→4I9/2 (914 nm) is about one order of magnitude weaker than the transition 4F3/2→4I11/2 (1064 nm). The former corresponds to a quasi-three level system, in which the lower energy level coincides with the ground electronic state. It is difficult to create and maintain population inversion in such a system when a strong transition, e.g. 4F3/2→4I11/2, is present in the same medium. One way to resolve this problem is to use a separate high power single mode or multimode semiconductor laser as the source of 914 nm line. An example of such systems was demonstrated by Johansson et al. in Optics Express Vol. 13, No. 7. Another way to reduce the gain competition effect is adjustment of mirror reflectivities and/or alignments. Dual-wavelength lasers based on this scheme to obtain CW blue radiations were recently reported by Herault et al. in Optics Express Vol. 13, No. 15. However, their architectures are complicated, requiring many optical components, and the efficiency is low.
Other challenges for intracavity frequency doubled lasers or intracavity SFM lasers include reduction of optical noises such as amplitude fluctuations in the green output caused by nonlinear interactions of the longitudinal modes. Three solutions to the “green problem” have been investigated in the prior art: (1) lasers operated with a great many longitudinal modes (˜100) to average out intensity fluctuations in time domain; (2) single longitudinal mode (SLM) lasers to eliminate longitudinal mode coupling; (3) lasers operated at few modes with decoupled eigenstates of polarization. All of these approaches require the insertion of mode-selection elements and/or temperature stabilization or a long resonator, introducing additional complexity and cost.
It would be an advantage and, in fact, an object of the present invention as well, to provide a method whereby a variety of wavelengths that are not available from a single laser diode or a DPSS laser or an ultra-compact laser based on the intracavity SFM or second-harmonic generation (SHG) schemes described in the prior art can be obtained at low optical noise.